In general, inkjet printers include at least one printhead that ejects drops of liquid ink onto a surface of an image receiving member. In an indirect or offset printer, the inkjets eject ink onto the surface of a rotating image receiving member, such as a rotating metal drum or endless belt, before the ink image is transferred to print media. In a direct printer, the inkjets eject ink directly onto print media, which may be in sheet or continuous web form. A phase change inkjet printer employs phase change inks that are solid at ambient temperature, but transition to a liquid phase at an elevated temperature. Once the melted ink is ejected onto the media or image receiving member, depending upon the type of printer, the ink droplets quickly solidify to form an ink image.
Inkjet printers are used to print a wide range of documents using various types and colors of ink. Some printed documents are read by both humans and machines. For example, a check includes printed text that is both human readable and readable by automated check processing equipment. Check processing machines use Magnetic Ink Character Recognition (MICR) to identify printed characters in a check, such as routing and account numbers, quickly and accurately. The magnetic ink readable by MICR machines includes a suspension of magnetic particles, such as iron oxide, which are detectable using a magnetic field. The use of MICR printing is widespread and enables automated processing of checks and other documents. Automated check processing machines perform high-speed character recognition using printed magnetic ink characters to identify account and routing numbers. While check processing is one application of magnetic ink printing, magnetic inks can be incorporated in a wide range of printed documents and can be used in conjunction with non-electromagnetic inks as well.
Printing with magnetic inks can be difficult, particularly in printing systems that use printheads having inkjets. In many cases, the ink supplied to a printhead that ejects magnetic ink is held in an internal reservoir. If an image that includes features to be printed with magnetic ink is printed with irregularity, the magnetic particles in the ink in the reservoir or other channels within the printhead can settle out of the ink. This settling makes getting enough magnetically active particles into the ink and eventually into the printed image troublesome. Additionally, media surface preparation or the fusing or spreading of other ink types on the media can interfere with the magnetic ink properly bonding to the media. Making magnetic ink printing in inkjet printing systems more efficient would be advantageous.